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Glial cells comprise about 80% of the cells in the human brain. But physiological models of EEG and fMRI often relate only to neuronal activity.

How do glial cells affect large-scale imaging modalities?

Also, does glial activity artifact dye imaging and local field potentials?

Are microscale recordings more sensitive to artifacts due to glial activity than macroscale recordings?

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    $\begingroup$ This is a good question but I want to address something. Neither EEG nor fMRI measure chemical neuronal activity via neurotransmitters. EEG measures synchronized electrical activity mostly in cortical neurons and fMRI measures cerebral blood flow which is assumed to be coupled with neuronal activity. I am almost certain that glial cells are not sources of noise, but I will refrain from writing an answer until I have sufficient links to provide in support of this. $\endgroup$ – Lazaros Mitskopoulos Dec 21 '14 at 21:35
  • $\begingroup$ I was being overly presumptuous. Fixed. $\endgroup$ – Malz Dec 22 '14 at 0:37
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I'm only going to attempt to answer a small part of your question: how does glial activation affect the blood oxygen level-dependent (BOLD) response measured by fMRI?

Schulz et al. (2012) were able to investigate the role of glial signaling on fMRI activity by simultaneously measuring neuronal responses with invasive optical imaging and fMRI. They found that glial activation prolonged the BOLD response, as shown in this figure:

figure from Schulz et al.

Here's how they interpret this result:

Although these glial responses did not initiate vessel dilations, they were associated with prolonged BOLD signals, suggesting a possible contribution to persistent vascular responses.

In other words: they hypothesize that the activity of the glial cells prolongs the dilation of the blood vessels, which is detected as a prolonged BOLD response.

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    $\begingroup$ very interesting! Are glial cells uniformly dense throughout cortex and/or white matter? Also, is there something akin to an HRF for glial cells that could be subtracted from the typical HRF to obtain a signal that better reflects non-glial activity? $\endgroup$ – blz Dec 22 '14 at 22:09
  • $\begingroup$ Nice answer! Thanx! Why would glial cells prolong BOLD responses? (+1, just probing :-) $\endgroup$ – AliceD Dec 23 '14 at 13:09
  • $\begingroup$ I think the answer addresses that, but let me know if I should clarify. The hypothesis is that glial cell activation causes the blood vessels to stay dilated longer than they would otherwise. $\endgroup$ – Josh de Leeuw Dec 23 '14 at 13:31

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